1. Field of the Invention
The present invention relates to a sheet feeding device adapted for use in computers, copying machines, printers, facsimile apparatus, word processors, typewriters or the like, and more particularly to a sheet feeding device utilizing a travelling vibration wave.
2. Related Background Art
A sheet feeding device utilizing travelling vibration wave has been already disclosed for example in the Japanese Laid-Open Patent Sho 59-177243. In said device, the sheet feeding is achieved by supporting a sheet between a pair of vibration members composed of an elastic material and pressed each other by a suitable pressure, and generating travelling vibration waves in said vibration members. On each vibration member there is adhered or pressed two groups of electromechanical energy converting elements such as piezoelectric elements, which are arranged with a phase difference of an odd multiple of .lambda./4 between said two groups, and, within each group, with a pitch of .lambda./2 and with alternating expansion-contraction polarity. One of said groups of piezo-electric elements on each vibration member is given an AC electric field of a frequency in the vicinity of specific frequencies of the vibration members (in practice the specific frequency of either vibration member), while the other of said groups of piezo-electric elements on each vibration member is given said AC electric field with a .pi./2 phase shift achieved by a .pi./2 phase shifter, thereby generating travelling vibration waves in said vibration members symmetrically with respect to the sheet feeding plane. A travelling vibration wave in each vibration member causes an elliptic movement in each point on a surface of said member opposite to the piezo-electric elements, and the sheet feeding is achieved by said elliptic movement of each point on mutually opposed faces of the vibration members, symmetric with respect to the sheet feeding plane, through friction between the sheet and the vibration members, and a change in the sheet feeding speed can be achieved for example by a change in the amplitude of the travelling vibration waves generated in the vibration members.
FIG. 3 schematically shows the principle of generating the sheet feeding force by such travelling vibration wave, in which a sheet 9 is pinched with a suitable pressure between vibration members 1, 5 in which travelling vibration waves are generated as illustrated. Each point on the surface of the vibration members 1, 5 moves generally along an elliptic trajectory. In the vibration member 1, when the travelling vibration wave moves to the right, as indicated by a white arrow, the surfacial point moves clockwise along said elliptic trajectory. As the travelling vibration waves generated in the vibration members 1, 5 mutually have a spatial phase difference of 180.degree., the projecting portions of said waves toward the sheet 9 always proceed with mutually opposed relationship. Since each point on such projecting portions moves in a direction opposite to the proceeding direction of the vibration waves in the members 1, 5, there is generated a sheet feeding force to the left, as indicated by a black arrow. In the recessed portions, there is generated a sheet feeding force in a direction the same as the proceeding direction of the travelling vibration waves, but such feeding force is weaker because of the smaller friction between the sheet and the vibration members resulting from a lower pressure in such recessed portions than in the projecting portions. Thus the total sheet feeding force is in a direction opposite to the proceeding direction of the travelling vibration waves.
However, in such conventional sheet feeding device, the travelling vibration waves are generated in both vibration members with AC voltages of a same phase even when the specific frequency of the vibration member varies individually for example by fluctuation in the sheet thickness or variation in the environmental conditions. Consequently a spatial phase aberration or deviation may be generated between the vibration waves of both vibration members as shown in FIG. 4, due to a change in the load or in the environmental conditions, thus resulting in a fluctuation in the sheet feeding amount.